Numerous pre-clinical and clinical studies, including our own, demonstrate that DC-based tumor immunotherapies can induce potent tumor specific immunologic and clinical responses. However, complete clinical responses have been rare, and the improvement of DC-based immunotherapies is the focus of considerable effort. Increasing evidence suggests that tumor induced immunosupression, and the presence or induction of regulatory T-cells (Tregs), may limit the efficacy of anti-tumor immunization strategies. Our hypothesis is that Tregs play a critical role in limiting DC-based immunization against tumors, and that the reduction/removal of Tregs prior to immunotherapy may result in enhanced anti-tumor immunity and improved therapeutic efficacy. The studies we propose here will provide the experimental foundation for this approach and enable a direct test of this hypothesis through clinical trials designed to evaluate combined immunization/Treg depletion approaches for the immunotherapy of patients with advanced stage cutaneous T-cell lymphoma (CTCL). In our preliminary studies, we have developed and evaluated a DC-based immunization strategy for the immunotherapy of patients with CTCL. To construct an autologous tumor vaccine, we have utilized patientderived matured and polarized DCs loaded with autologous circulating tumor cells from leukemic CTCL Patients. Our preliminary data demonstrates that immunization of a patient with end-stage Sezary Syndrome SzS resulted in significant anti-tumor immune responses and a complete clinical response. The studies we propose here are designed to continue the logical progression of the development of this immunization strategy by identifying and obviating immunosuppressive mechanisms in the host that are likely to limit vaccine efficacy. Our preliminary data demonstrate that CTCL patients have elevated levels of Tregs compared to normal controls. In this project we will evaluate and characterize Treg populations in CTCL patients, and will develop strategies to reduce or eliminate Tregs prior to immunization using already approved chemotherapeutic agents. These studies will provide the basis for the in vivo evaluation of the effectiveness of combined immunization and Treg depletion therapies for CTCL, and future analogous approaches for the treatment of human cancers.